From: Jeremy Fitzhardinge <jeremy@goop.org>
Subject: Re: [PATCH RFC V2 3/5] jump_label: if a key has already been initialized,
 don't nop it out
Date: Wed, 05 Oct 2011 17:47:21 -0700
Message-ID: <4E8CFA99.8030805@goop.org>
References: <cover.1317506051.git.jeremy.fitzhardinge@citrix.com> <477dead9647029012f93c651f2892ed0e86b89e7.1317506051.git.jeremy.fitzhardinge@citrix.com> <20111003150205.GB2462@redhat.com> <4E89E28C.7010700@goop.org> <20111004141011.GA2520@redhat.com> <4E8B3489.60902@zytor.com> <4E8CF348.4080405@goop.org> <4E8CF385.2080804@zytor.com>
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To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Jason Baron <jbaron@redhat.com>, Steven Rostedt <rostedt@goodmis.org>, "David S. Miller" <davem@davemloft.net>, David Daney <david.daney@cavium.com>, Michael Ellerman <michael@ellerman.id.au>, Jan Glauber <jang@linux.vnet.ibm.com>, the arch/x86 maintainers <x86@kernel.org>, Xen Devel <xen-devel@lists.xensource.com>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>, peterz@infradead.org
List-Id: xen-devel@lists.xenproject.org

On 10/05/2011 05:17 PM, H. Peter Anvin wrote:
> On 10/05/2011 05:16 PM, Jeremy Fitzhardinge wrote:
>> On 10/04/2011 09:30 AM, H. Peter Anvin wrote:
>>> On 10/04/2011 07:10 AM, Jason Baron wrote:
>>>> 1) The jmp +0, is a 'safe' no-op that I know is going to initially
>>>> boot for all x86. I'm not sure if there is a 5-byte nop that works on
>>>> all x86 variants - but by using jmp +0, we make it much easier to debug
>>>> cases where we may be using broken no-ops.
>>>>
>>> There are *plenty*.  jmp+0 is about as pessimal as you can get.
>> As an aside, do you know if a 2-byte unconditional jmp is any more
>> efficient than 5-byte, aside from just being a smaller instruction and
>> taking less icache?
>>
> I don't know for sure, no.  I probably depends on the CPU.

I was thinking about making the jump-label stuff generate a small jmp if
the offset is small (specifically "jmp; nop3", or perhaps "jmp; ud2a;
nop" to make absolutely sure there's no speculation beyond the jmp), on
the grounds that, while it probably doesn't matter for any modern
Intel/AMD processor, it may help for others.  But I couldn't find any
concrete evidence to support it, and there's already enough questions
about doing "live" code updates without adding more instruction patterns.

    J